Download Microbial Carbon Sequestration in the Ocean: The Big Role of Small

Vol.27 No.3 2013
Special Reports
Low Carbon Development
Microbial Carbon Sequestration in the
Ocean: The Big Role of Small Bugs in
Mitigating Global Climate Change
— An Interview with Prof. JIAO Nianzhi
Ocean is the largest carbon reservoir on Earth. By taking up about one-third of all human carbon
emissions, it has effectively reduced atmospheric carbon dioxide and eased global climate change.
Scientists have studied extensively on the mechanisms of CO2 absorption in the ocean, or the
so-called “carbon pumps”, from physical to chemical and biological pumps. Microbes, traditionally
regarded as organic matter decomposers, had been long neglected until a few years ago Prof. JIAO
Nianzhi from Xiamen University proposed the concept “microbial carbon pump (MCP)” illustrating that
these tiny marine inhabitants are also producers of refractory dissolved organic carbon, a persistent
form of matter which can be stored in the ocean for thousands of years and thus constitute an ideal
carbon pool. Today, the MCP concept has been widely accepted as a promising sequestration strategy,
and Prof. JIAO is working hand in hand with scientists from all over the world under the Scientific
Committee on Oceanic Research (SCOR) Working Group 134 to better understand how to use these
magic creatures for preserving carbon in the deep blue.
During an interview with BCAS reporter XIN Ling on June 4, Prof. JIAO introduced the rationale of
MCP and its advantages over other carbon pumps. He also envisioned the potential application of MCP.
In his words, “the problem we’re facing is huge, from a bug in the ocean to climate change on Earth”.
Prof. JIAO Nianzhi is an outstanding scientist in
microbial ecology and ocean carbon cycling.
Before joining Xiamen University as Cheung
Kong Chair Professor in 2000, he had conducted
research at the CAS Institute of Oceanology, the
MIT, the University of Tokyo and the National
Institute for Environmental Studies of Japan. He
was elected Member of the Chinese Academy of
Sciences in 2011. (Photo: BCAS)
BCAS: What is the scientific background of the MCP
theory?
Prof. JIAO: There are several known mechanisms
for the ocean to store carbon dioxide. One is called the
“solubility pump”. After carbon dioxide dissolves in
water, there is a natural physical process to transport
dissolved inorganic carbon from the ocean's surface to its
interior. However, many studies have confirmed that as
atmospheric CO2 continue to accumulate and heat up the
Earth, the solubility of CO2 in the ocean will decrease and
the reservoir becomes less effective. For instance, in the
last century, the solubility pump contributed to around 80%
of all anthropogenic CO2 uptake by the ocean. Today the
contribution rate is less than 50%. Besides, the chemical
reactions between CO 2 and water have led to ocean
acidification and many negative consequences for marine
ecosystems such as the death of coral reefs.
A second mechanism is called the “carbonate pump”.
Bulletin of the Chinese Academy of Sciences
151
BCAS
Vol.27 No.3 2013
Low Carbon Development
When marine organisms die, a fraction of their calcium
carbonate skeletal coverings will be transported to the
bottom of the ocean and buried there for long term storage.
However, a formerly ignored fact is that in forming calcium
carbonate, the organism will also release a same equivalent
of carbon dioxide as a result of biochemical reaction, and
heterotrophs are a releaser rather than absorber of carbon.
Now the mechanism is often referred to as the “carbonate
counter-pump”.
A third mechanism, the “biological pump”, is well
known and can be manipulated for human purpose. A
famous case is iron fertilization. We all know there are very
few nutrient substances in the barren ocean. If we introduce
proper trace elements into the ocean, they can remarkably
enhance the ocean’s biological productivity and gear up
the biological pumps. The notion was first proposed in the
late 1980s by US oceanographer John Martin, the man
who declared that “give me half a tanker of iron and I’ll
give you the next ice age”. However, iron fertilization will
induce phytoplankton blooms and could trigger marine
ecological disasters including red tide, the death of fishes
from toxic materials, as well as invisible but fatal changes
in the structure and function of the marine community. Iron
fertilization has also been proven to indirectly accelerate
ocean acidification.
The “microbial carbon pump” or MCP, as we proposed,
refers to the transformation of dissolved organic carbon
(DOC) in the water from labile to refractory states driven
by microbes including bacteria, archaea, phytoplankton
and viruses. The labile DOC is biologically available while
refractory DOC is hard to digest and thus accumulate to a
huge carbon reservoir, constituting carbon sequestration in
the ocean.
A diagram of the microbial carbon pump (MCP, yellow color) and
the classical biological pump (green color) as well as relevant
biological processes involved in carbon cycle in the ocean. (Photo
courtesy Prof. Jiao Nianzhi)
152 Bulletin of the Chinese Academy of Sciences
BCAS: It must be hard to understand at first: in
people’s intuition, microbes are decomposers that give
off carbon dioxide to the air. How can they store carbon
instead?
Prof. JIAO: Yes, it was difficult in the beginning.
A key fact here is when microbes decompose marine
organisms for food, they also have to protect themselves
from being degraded by extracellular enzyme present in
the marine environment. By synthesizing D-amino acids,
microbes build up very strong cell walls to resist the
degradation of enzymes. When they die, the D-amino acids
in them are released into the ocean and become refractory
dissolved organic carbon (RDOC), which is left unused
and thus accumulated over time to form a great carbon
reservoir. This new role of microbes, totally different from
its traditional one as decomposers, is very important in
understanding the MCP.
BCAS: How did such a novel idea occur to you at first?
Prof. JIAO: It was about ten years ago when my
group was looking into the global distribution of aerobic
anoxygenic phototrophic bacteria (AAPB), an old group
of bacteria with newly discovered ecological significances.
We came to a conclusion that was opposite to that in the
literature. So we spent a lot of time digging into different
methods and data sets, and finally revealed the specific
linkage between AAPB and DOC. As a result, the MCP
concept was developed which then began to receive more
and more recognition from around the world.
BCAS: What are you and your coworkers doing in
SCOR Working Group 134?
Prof. JIAO: Together we are making difference. We
have put efforts to perfect and promote the MCP framework.
We are also developing the MCP, with a multidisciplinary
strength pooling the best biologists, chemists and
paleoclimate experts in the field together. Now, our group
is a consortium of 26 scientists from 12 countries. By far
we have convened three WG meetings, in Xiamen, San
Juan and Delmenhorst respectively, as well as a number of
independent workshops and sessions.
BCAS: How is the MCP superior to other carbon
pumps?
Prof. JIAO: A big difference is that the MCP is nonsinking, while physical and biological pumps more or less
depend on the sinking process. The solubility pump is
dependent on the transportation of water mass from surface
to depths. The biological pump depends on the sinking of
particulate organic carbon in the water column. In contrast,
Vol.27 No.3 2013
Special Reports
Low Carbon Development
Prof. Jiao at discussion with SCOR WG members. (Photo courtesy Prof. Jiao Nianzhi)
the MCP is non-sinking so carbon can be kept at any depth
of the water, even in surface waters. More importantly, the
MCP is a naturally occurring process that works at very
low cost and without any known negative biological or
ecological consequences.
The MCP has been used to explain many mysteries.
For instance, now we know that 95% of organic carbon in
the ocean is in the form of dissolved organic carbon (DOC)
and 95% of the DOC is recalcitrant DOC (RDOC). The vast
existence of RDOC explains the gap between the ocean’s
huge carbon storage and the limited amount of particulate
organic carbon known on the sea floor.
The MCP is also the key to accounting for the paradox
that some marine areas are rich in organic carbon but poor
in biological productivity. That’s because the organic carbon
produced by phytoplankton is labile and can be easily
converted to carbon dioxide. Only carbon that goes through
the MCP can become refractory and stable in the marine
environment.
A piece of evidence for the MCP comes from
paleoclimate study. During the Proterozoic, our planet was
ruled by bacteria and the oceans held 500 times as much
DOC as today, which was most likely generated by the
MCP. The dramatic increase of carbon in the ocean and
decrease in the atmosphere had exacerbated climate changes
to the biggest ice age called the “snowball Earth” .
BCAS: Where does the RDOC go eventually?
Prof. JIAO: Some of the RDOC evolve into
aggregates and finally sink to the bottom of the ocean.
Some rise to the water surface amid ocean circulation to
be degraded by ultraviolet light. As long as the marine
environment is stable and free from major natural or human-
induced perturbations, the RDOC will be accumulated
over time. The average age of RDOC in the current ocean
is about 5,000 years. From this perspective, the ocean is
endowed with much higher carbon sequestration efficiency
than terrestrial carbon sinks, by separating CO2 from the
atmosphere for as long as possible.
BCAS: How do you see the potential applications of
MCP?
Prof. JIAO: The MCP is especially applicable to
waters with eutrophication problems, like the coastal oceans
of China. In such environment, the biological pump does
not fit in because it will facilitate ecological disasters like
harmful algal blooms and other invisible threats. What we
should do is to reduce the nutrient level in the ocean by
curbing chemical fertilization on the land, and develop the
MCP for carbon sequestration.
The MCP can also play a part in the carbon sequestration
in estuarine waters. The sinking-process-based biological
pump is inefficient due to the shallow water depth, the strong
mixing process and the severe resuspension of particles in
estuarine waters. The MCP is not confined to such physical
conditions thanks to its non-sinking nature.
I think an integrative consideration of the biological
pump and the MCP will help us identify the responses
of marine carbon sinks to climate changes, as well as the
practices conducive to carbon sequestration in the ocean.
The problem we’re facing is huge, from a bug in the
ocean to climate change on Earth. I’m looking forward to
more communications and collaborations with scientists
from all related fields. I also hope that we can find a
way to convey these ideas to policymakers and seek for
implementation opportunities as soon as possible.
Bulletin of the Chinese Academy of Sciences
153